Recap I Lecture 41 Matthias Liepe, 2012

Transcription

1 Recap I Lecture 41 Matthias Liepe, 01

2 Recap II

3 Nuclear Physics The nucleus Radioactive decay Fission Fusion Particle Physics: What is the Higgs? Today:

4 Nuclear Physics: The Nucleus Positive charge and most of the atom s mass are concentrated in a tiny dense core of ~ m to m in diameter. -> Atomic nucleus Nuclei are composed of protons (charge = +e per proton) and neutrons (no electric charge) Z = # of protons N = # of neutrons Atomic mass number: A = Z + N

5 Most nuclei are spherical (some are ellipsoidal): R R 0 A 1/3 -> Effective radius: where R m = 1. fm (1 femtometer = 1 fermi = 1 fm = m ) Element type and chemical properties are determined by Z. Different species (as determined by Z and A) are called nuclides.

6 Notation to describe nuclides: A Z X or A X chemical symbol of element Examples: 4 He O Fe 79Au Nuclides with same Z but different A (i.e. different N) are isotopes of each other. Examples: He 3 4 He

7 Plot of known Nuclides: The black shading indicates the band of stable nuclides. Low-mass, stable nuclides have essentially equal numbers of neutrons and protons. More massive nuclides have an increasing excess of neutrons.

8 Plot of known Nuclides: Most nuclides are not stable and undergo radioactive decay by emitting radiation and transferring into other nuclides. There are no stable nuclides with Z>83 (bismuth).

9 Radioactive Radiation Types: Alpha () particles - He nuclei (positively charged). - Can be stopped by a thick piece of paper or several cm of air. Beta () particles - Electrons (negatively charged) or positrons (anti-electrons; positively charged). - Can penetrate several sheets of paper, thin metal foils, 1 m of air. Gamma () - Very high energy photons (uncharged). - Could pass through a human hand. - Stopped by several cm of lead.

10 Radioactive Decay: Most nuclides are not stable & undergo radioactive decay by emitting radiation & transforming into other nuclides. Radioactive decay is a statistical process Decay constant : = probability that a particular nuclide will decay in a unit time interval; [] = 1/s = fraction of nuclei in a large sample that are expected to decay on average per unit time interval has a characteristic value for every radionuclide. is independent of any external influence, including the decay of another nucleus.

11 Radioactive Decay: If a sample has N radioactive nuclei of a given type then the average number decaying per unit time is N. Define decay rate R as: R dn dt N average number of decays per time Integrate to number of radioactive nuclei vs. time: N( t) t N(0) e N(0) with N(0) = number of radioactive nuclei at t = 0 Number of radioactive nuclei decreases exponentially with time constant: e t 1

12 N/ N Radioactive Decay: Half Time T 1 The half-life is the time at which half of the original sample remains: N( T ) 1 T1 e N(0) 1. T 1 T 1 ln() Can rewrite the number of radioactive nuclei vs. time equation: N t ln( ) T ( t T 1 ln() ( t) N(0) e N(0)( e ) N(0) N(0)( ) t T ) t T 1

13 Application: carbon-14 dating: 14 Carbon-14 ( 6C) is an unstable isotope of carbon ( T years ) which is produced in the upper atmosphere 14 by cosmic ray neutrons colliding with N : 14 7N n 6C 14 This 14 C is rapidly oxidized to 14 CO and thus can enter living organisms through photosynthesis & the food chain. 14 [ CO] 1 In the atmosphere, [ CO ] A living organism that derives its carbon from the atmosphere will have the same [ 14 C][ 1 C] in its tissues. But once the organism dies it stops taking in carbon & the amount of 14 C in its tissues decreases due to radioactive decay: 14 6 C 14 7 N 7 p

14 Application: carbon-14 dating: By measuring the ratio of [ 14 C][ 1 C], it can be determined how long it has been since the organism died. Because T years, the 14 C dating method is good for ages 50,000 years. 14 C dating may be complicated because the proportion of 14 C in the atmosphere has not been constant. So, other dating techniques are often used as calibrations for 14 C dating. Modern human activity has altered the [ 14 C][ 1 C] in the atmosphere through nuclear weapons tests & the burning of fossil fuels.

15 Measuring Radiation: Geiger tube radiation detector (Geiger counter) Radiation (alpha particles, beta particles, or gamma ray photons) will cause electrons to be ejected from the gas or the metal in the tube. That electron will then cause more ejections -> number of electrons is multiplied by a factor of about 10 6 to 10 8 before reaching the thin wire. Electrons create a current in the thin wire at the center of the counter.

16 Radiation Dosage: Absorbed dose = radiation energy absorbed by an object per unit mass Units: 1 grey = 1 Gy = 1 J/kg = 100 rad Example: Radiation dose from natural sources per year: ~ mgy = 0. rad

17 The Nuclear Force: Protons repel each other because of their charge (electric force). A totally different attractive force must bind protons & neutrons together in the nucleus. This nuclear force is thought to be a secondary effect of the strong force that binds quarks together to form neutrons & protons. The nuclear force must be a very short range force because its influence does not extend far beyond the nuclear surface. The atomic mass unit: The atomic mass unit, u, is chosen so that the atomic (not nuclear) mass of 1 C is exactly 1 u. 1u kg. 1u MeV/ c Atomic mass is often reported in these atomic mass units..

18 M the mass of a nucleus. m i i the total Nuclear binding energy: mass of its individual protons & neutrons. M ( m c ). m i, or, Mc i The binding energy, E BE, of the nucleus is: E BE i ( m c i ) i i Mc It is the energy that would be required to separate a nucleus into its component nucleons. The binding energy per nucleon, E BEn :. E BEn E A BE

19 binding energy per nucleon, E BEn [MeV] The "curve of binding energy": A graph of binding energy per nucleon of common isotopes. More tightly bound. The Nickel nuclide 6 Ni has the highest binding energy per nucleon. Number of nucleons in nucleus (A)

20 Nuclear reactions: Conserved quantities are electric charge & total number of nucleons. The energy Q released in a reaction is: Q mc i m c f mc where m i is the total mass of the reactants and m f is the total mass of the products., Recall: E = mc, so can convert mass to energy! Q 0 when some mass is converted to energy.

21 Nuclear reactions: Example Alpha decay of 38 U: U 90Th Atomic mass of Atomic mass of Atomic mass of U Th He u u u m m f - m i ( u u) u u Q mc ( u) c MeV 1u c 4.5 MeV Q>0, so energy is released Almost all of this energy released is kinetic energy of the particle. (Why?) (U: Uranium; Th: Thorium; : helium nucleus)

22 binding energy per nucleon, E BEn [MeV] Nuclear fusion: light nuclei combine to form a larger nucleus (e.g. in stars) The "curve of binding energy": Nuclear fission: large nucleus is converted to smaller nuclei plus energy (e.g. in nuclear reactor) Number of nucleons in nucleus (A)

23 Nuclear Fission: Large nucleus smaller nuclei, neutrons, & energy Example: U n U (excited state, unstable) one possibility U Ba Kr Induced nuclear fission event: A neutron is absorbed by the nucleus of a uranium- 35 atom, which in turn splits into fast-moving lighter elements (fission products) and free neutrons. More neutrons produced than consumed -> chain reaction. (U: Uranium; Ba: Barium; Kr: Krypton) n

24 Nuclear Reactors

25 Drawing of the first artificial reactor, Chicago Pile-1 CP-1 was built on a rackets court, under the abandoned west stands of the original Alonzo Stagg Field stadium, at the University of Chicago. The first self-sustaining nuclear chain reaction was initiated in CP-1 on December, 194.

26 Nuclear Fusion: Two or more small nuclei single heavier nucleus, other particles, & energy. Example: 3 4 1H 1H He n energy Fusion of deuterium with tritium creating helium-4, freeing a neutron, and releasing MeV of energy

27 Natural Fusion Reactor: The Sun

28 Artificial Fusion Reactors Transformer coil Poloidal Plasma field Toroidal magnet field magnet In 1997, the Joint European Torus produced a peak of 16.1 megawatts (1,600 hp) of fusion power (65% of input power), with fusion power of over 10 MW (13,000 hp) sustained for over 0.5 sec.

29 Particle Physics: The Standard Model (courtesy of CERN)

30 What is the Origin of Mass? Fundamental particles do not have any size. Here the different sizes represent the different masses. The masses of neutrinos are so small they would not be visible at this scale. Why do fundamental particles have such different masses? How do particles gain mass? To explain these mysteries, theories predict a new particle, the Higgs particle. (image courtesy of CERN)

31 The Higgs Field Theory To understand the Higgs mechanism, imagine that a room full of physicists chattering quietly is like space filled with the Higgs field A well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field. (cartoons courtesy of CERN)

32 ... if a rumor crosses the room, it creates the same kind of clustering, but this time among the scientists themselves. In this analogy, these clusters are the Higgs particles. See also: (cartoons courtesy of CERN)

33 What is the Universe made of? Faster expansion rate is attributed to a mysterious, dark energy / force that is pulling galaxies apart. Gravitational lensing and galaxy rotation speeds indicates the presence of dark matter (image courtesy of NASA)